Nitration of cellulose



Patented Apr. 15, 1941 .UNETE TENT OFFICE NITRATION OF CELLULOSE No Drawing. Application June 20, 1939, Serial No. 280,081

4 Claims.

This invention relates to an improved nitrocellulose and method of preparation thereof and more particularly it relates to an improved methanol-soluble nitrocellulose and method of preparation thereof.

Methanol-soluble nitrocellulose is nitrocellulose having an average nitrogen content from 11.5% to 12%. This material is used in preparing films by casting from solution in a solvent consisting essentially of methanol. It is made by nitrating cellulose for 15-45 minutes in a large amount of nitrating acid, say 40 to 50 parts of acid per part of cellulose, the acid usually consisting of about 16-18% water, about 20-30% nitric acid altho a higher percentage may be used, and the remainder sulphuric acid plus a little nitrosyl sulphuric acid. The nitrating acid is prepared from the spent acid of previous nitrations, usually by fortifying with strong acids, since reuse of the acid is essential for economic reasons because of the large quantity required in each nitration.

Nitrocellulose completely soluble in methanol has never been available. For complete methanol-solubility, all portions of the nitrocellulose must have a nitrogen content in a very narrow range. An average nitrogen content in the desired range is readily obtained, but, due to inhomogeneity, methanol solutions of the nitrocellulose have contained insoluble and partly soluble particles. It has been thought that this inhomogeneity has been due to incompletely nitrated particles since it has long been known thatlaboratory nitrations contain such particles. Because of the incomplete solubility of the nitrocellulose, it is .necessary to filter the solutions before casting films therefrom. Rapid plugging of the filters makes this operation expensive and, as a result, the quality of the product has come to be judged by its filterability. Filterability is expressed in terms of filterability value determined by a standard test hereinafter described; the higher the filterability value the better the product.

It has been believed that poor filterability is caused chiefly by incompleteness of nitration. Attempts to improve it by nitrating at higher temperatures, or for a longer time, or with acids richer than usual in nitric acid, etc., have been made, without, however, obtaining a nitrocellulose with a high filterability value.

.It is an object of this invention to provide a homogeneous nitrocellulose completely soluble in methanol. Another object is to provide a methanol-soluble nitrocellulose of greatly improved filtering characteristics, i. e. having a high filterability value. Still another object is to provide an economical method of preparing this improved nitrocellulose.

Now in accordance with this invention, I have found that a methanol-soluble nitrocellulose of greatly improved filterability can be obtained by a simple and expedient method. My new nitrocellulose is characterized by homogeneity and smoothness in methanol solution and is especiallyv distinguished by its high filterability value.

I prepare my improved nitrocellulose, in accordance with this invention, by a method in which the time, temperature, quantity of acid, and composition of acid are essentially those known in the past. I use spent acid to prepare my nitrating acid as in the past. However, I utilize only nitrating acid free from suspended cellulose nitrate fines which have nitrogen content in excess of 12.0%. For example, the nitrating acid is prepared from spent acid obtained only from previous nitrations in which the product was nitrocellulose of less than 12% nitrogen content. This spent acid is reused promptly, fortifying it if necessary.

After nitration, I purify and stabilize the nitrocellulose, digesting it if desired, dehydrate and so forth, in the usual manner.

The product which I obtain has a filterability value much higher than that previously obtained and its methanol solutions are thus much more economically filtered. Filterability or solubility of methanol-soluble nitrocellulose is measured by a standard test given in detail by Baker, Morgan, and Quaid in their U. S. Patent 2,105,627, page 2, lines 43-74. By the term, filterability value, used herein and in the appended claims, is meant the numerical result of this test. Briefly, it is the number of grams of nitrocellulose (on the dry basis) which can be forced under specified conditions through one square centimeter of a certain filter paper, when in the form of a solution consisting of 24 parts nitrocellulose, 72.5 parts methanol, and 3.5 parts of camphor, before flow ceases due to plugging of the filter by incompletely dissolved material. I-Ieretofore, nitrocellulose with a filterability value of 5 has been considered very satisfactory.

Now the product obtained according to this invention has a new high filterability value above at least 8, say from 8 to about 20 and, under preferable conditions of preparation, it has a filterability value of from about 16 to about 20. The product is in the usual fibrous form of nitrated cellulose. It has a nitrogen content from about 11.7% to about 12.0%, but preferably the nitrogen content is in the range from about 11.80% to 11.95%.

In attempting an explanation of the improvement obtained, I proceed upon the belief that, contrary to the old idea that poor filterab-ility is caused chiefly by incompletely nitrated cellulose, poor filtera'bility is really largely due to fines ofcellulose, nitrated to a higher nitrogen content than the methanol-soluble range. It will be understood that spent acid, which is recovered by centrifuging from the nitrated cellulose, invariably contains nitrocellulose fines. In nitrating processes in which spent acid from nitration of cellulose to various nitrogen contents is conveyed to a common storage, it is obvious that fines nitrated to above 12% nitrogen content will be present in the stored acid. However, even when spent acid from nitration to a nitrogen contentof less than 12%, for example, to a methanol-soluble grade,has been segregated and stored separately, its use has led tothe production of material of poor filterability. I have found that, surprisingly, fines in spent acid from nitration to the methanol-soluble grade become further nitrated to above 12% nitrogen upon standing in the acid over a period of time. I therefore reuse spent acid promptly instead of allowing it to accumulate until adjustment of its composition is convenient, as in past practice. Thus, I use nitrating acid containing no nitrated cellulosic fines of about 12% nitrogen content.

In the production of my improved nitrocellulose, I first nitrate a quantity of cellulose to a nitrogen content in the range from 11.7% to 12.0% nitrogen using an acid of suitable composition prepared as usual from the usual spent acid, that is, spent acid which has been stored for an indefinite period and which may have been previously used in nitrating to any nitrogen content. The nitrocellulose so obtained is not of the high standard of methanol-solubility according to the present invention and it is therefore'sold for some other use. The newly formed spent acid is recovered by centrifuging the nitrocellulose as usual and is then promptly fortified and utilized in the nitration of cellulose to the product according to this invention. The spent acid is fortified and reused for nitration within 12 hours of its recovery from thepreviousnitrationbut preferably it is fortified and reused Within 6 hours or less, in general reusing it as soon as possible. Prefer-p ably, the acid is used as soon as possible after it has been fortified. Preferably also, the acid is keptcool until immediately before use in nitration. The spent acid is fortified to a composition known to the art to nitrate cellulose to a nitrogen content in the range from about 11.7 to about 12.0%, preferably to a nitrogen content in the range from about 11.80% to about 1.95% as best filterability results are obtained in this preferred nitrogen range. A wide range of such known acid compositions and known nitrating conditions are suitable, but it will be understood that the composition must b adjusted carefully I and accurately so to insure nitration within the range of nitrogen contents hereinabove mentioned.

After the cellulose is nitrated, it is recovered from the acid, quickly drowned, Washed, and stabilized in the usual manner. It is digested to reduce its viscosity characteristic to the level desired for its later application, for example, to a level suitable for preparing dopes which may be conveniently filtered and cast to form films.

Digestion improves the filterability value, the improvement varying with the reduction in viscosity produced. However, the strength and toughness of the film depends to some extent upon the viscosity characteristic and the nitrocellulose is, therefore, digested until it attains a viscosity characteristic level in the well-known and usual range for film nitrocellulose. The nitrocellulose may then be dehydrated in the usual manner.

It will be noted that the spent acid used in the hereinabove described method of preparing my improved nitrocellulose had been previously used in nitrating cellulose to a nitrogen content of 11.7 to 12.0%. However, spent acid previously used for nitrating cellulose to any nitrogen content below 12% may be utilized. Spent acid which has been used in nitrating cellulose to a relatively low nitrogen content, for example, 11.0% may be allowed to stand for a somewhat longer period than acid which has been used in nitrating to 11.7% to 12.0% nitrogen, although preferably it is reused promptly. The temperature at which the acid kept will alsoinfiuence the time for which it may be kept, higher temperature such as from 30-50 C. calling for more grompt reuse than lower temperatures, say from The improved nitrocellulose according to my invention may be prepared by an alternative method. In this method I also utilize a nitrating time, temperature, quantity of acid, and compo- .sition of acid essentially the same as those used in the past. However, I treat the spent acid used to make up the nitrating acid, either before or after fortifying it, to remove all suspended cellulosic matter, for example, by filtration. Spent acid which has been stored for an indefinite period and which may have been used in nitrating to any nitrogen content may be utilized in this method.

The examples following are given further to illustrate my invention.

Example 1 Percent Nitric acid 25.51 Sulphuric acid 51.75 Nitrosyl sulphuric acid 5.04 Water 17.70

1900 pounds of this acid, warmed to 44 C., were placed in the usual nitrating pot equipped with agitator and forty-two pounds of purified cotton linters dried to 0.44% moisture were immersed therein. The total time elapsed between the recovery of the spent acid from its previous nitration to the immersion of the cellulose in the fortified acid was 5 hours. The mixture was agitated for 20 minutes and then conveyed to a centrifuge where the spent acid was removed from the nitrocellulose. The Y nitrocellulose was then drowned, stabilized, and washed in the usual manner. To secure maximum usefulness of the product in methanol solutions, it was then digested in hot water under pressure in the manner well known to the art for reducing viscosity. The digested material had a viscosity of 8.4 seconds as determined according to A. S. T. M. specifications D--'301-'-33, formula, A. Its nitrogen content was 11.84%. l

The product obtained gave brilliant smooth solutions in methanol. Its filterability value determined as hereinbefore mentioned was 17.

Example 2 Percent Nitric acid 25.07 Sulphuric acid r 52.53 Nitrosyl sulphuric acid 4.65 Water 17.75

1900 pounds of this acid, warmed to 44 C., were placed in the usual nitrating pot equipped with agitator and forty-two pounds of purified cotton linters dried to 0.28% moistur were immersed therein. The total time elapsed between the recovery of the spent acid from its previous nitration to the immersion of the cellulose in the fortified acid was 3%. hours. The mixture was agitated for 20 minutes and then conveyed to a centrifuge where the spent acid was removed from the nitrocellulose. The nitrocellulose was then drowned, stabilized, and washed in the usual manner. To secure maximum usefulness of the product in methanol solutions, it was then digested in hot water under pressure in the manner well known to the art for reducing viscosity The digested material had a viscosity of 7.8 seconds as determined according to A. S. T. M. specifications D-301-33, formula A. Its nitrogen content was 11.89%.

The product obtained gave brilliant smooth solutions in methanol. Its filterability value determined as hereinbefore mentioned as 13.1.

Example 3 Per cent Nitric acid 31.67 Sulphuric acid 47. 06 Nitrosyl sulphuric acid 4. 02 Water 17. 25

1900 pounds of this acid, warmed to 44 C., were placed in the usual nitrating pot equipped with agitator and forty pounds of purified sheet sulphite wood pulp, dried to less than 1.5% moisture and chipped in the manner described in U. S. Patent No. 2,028,080 issued to R. L. Stern, were immersed therein. The total time elapsed between the recovery of the spent acid from its previous nitration to the immersion of the cellulose in the fortified acid was hours. The mixture was agitated for 46 minutes and then conveyed to a centrifuge where the spent acid was removed from the nitrocellulose. The nitrocellulose was then drowned, stabilized, and.

washed in the usual manner. To secure maximum usefulness of the product in methanol solutions, it was then digested in hot water underpressure in the manner well known to the art for reducing viscosity. The digested material had a viscosity of 7 seconds as determined according to A. S. T. M. specifications D-301-33, formula A. Its nitrogen content was 11.80%.

I The product obtained gave brilliant smooth solutions in methanol. Its filterability value determined as hereinbefore mentioned Was above at least 10.5.

Example 4 Spent nitrating acid taken from a central storage tank receiving spent acids from nitration of celluloseto a variety of nitrogen contents was adjusted to the following compositions:

Per cent Nitric acid 25.40 Sulphuric acid s 51.58 Nitrosyl sulphuric acid 5. 87 Water 17. 15

All suspended cellulosic matter was then removed from the acid by filtering through a deep bed of glass wool. 2720 parts by weight of this clarified acid, warmed to 40 C. were placed in the usual nitrating pot equipped with agitator and 60 parts by weight of purified cotton linters dried to 0.11% moisture were immersed therein. The mixture was agitated for 22 minutes and then conveyed to a centrifuge where the spent acid wasremoved from the nitrocellulose. The nitrocellulose was then drowned, stabilized, and washed in the usual manner. To secure maximum usefulness of the product in methanol solutions, it was then digested in hot water under pressure in the manner well known to the art for reducing viscosity. The digested material had a viscosity of 11 seconds as determined according to A. S. T. M. specifications D-301-33, formula A. Its nitrogen content was 11.85%.

The product obtained gave brilliant smooth solutions in methanol. Its filterability value determined as hereinbefore mentioned was above at least 9.7.

It will be understood that the details and examples given hereinbefore are illustrative only, and in no way limiting on my invention as broadly described hereinbefore and in the appended claims.

What I claim and desire to protect by Letters Patent is:

1. In the method for the preparation of methanol-soluble cellulose nitrate which comprises nitrating cellulose with a spent acid recovered from a previous nitration fortified to contain about 20-30% nitric acid, 16-18% water and the remainder sulfuric acid and nitrosyl sulfuric acid, conducting the nitration step with said acid freed of suspended cellulose nitrate having a nitrogen content in excess of 12%, whereby there is obtained at methanol-soluble cellulose nitrate of improved filterability, suitable for film preparation, and having a nitrogen content between 11.7% and 12%.

2. In the method for the preparation of methanol-soluble cellulose nitrate by nitration of cellulose with fortified spent acid obtained from previous nitrations, separating spent acid in which cellulose nitrate of less than 12% nitrogen content has been prepared from said cellulosenitrate,

fortifyingthe spent acid to. a composition of about 20-30% nitric acid, 16-18% Water, and the. remainder sulfuric and nitrcosyl sulfuric. acids,v

lulose nitrate of improved filterability, suitable for film preparatiomvhaving a nitrogen content between 11.7% and 12%.

3. In the method for the preparationiofmethanol-s'oluble cellulose nitrate by nitration of cellulose with fortified spent acid obtained from previous nitrations, separating spent acid from nitrated cellulose prepared therein, removing suspended cellulose nitratefines from the separated acid, adjusting the spent acid to a composition of about 20-30%,nitric acid, 16-18% water; and theremainder sulfuric and nitrosyl sulfuric. acids; suitable for the preparation of methanol-soluble nitrocellulose, and. nitrating cellulose therewith 1 whereby there :isrobtainedj a methanol-solublecellulose nitrate of improved filterability, suitable for film preparation, having a nitrogen content between 11.7% and 12%.

4. A fibrous nitrated cellulose capable of forming homogeneous smooth solutions in methanol and strong films from methanol solutions, said nitrated cellulose having afilterability value between 8' and about 20, a.- nitrogen content between 11.7 and 12%, and being substantially identical with the product formed by the process of. claim 1.

WILLIAM HARRY F. FRAVEL. 

